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Coal Ash Reservoir High-Hazard Dam Modification Case Study
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Coal Ash Storage Reservoir is a 300-acre, valley fill type reservoir that was constructed by building an approximately 130-foot tall dam across a valley to hold sluice water containing coal combustion residuals (CCR) from the operation of the nearby coalfired power plant.

The reservoir had to be capped in place with a cover system. The closure criteria required that there be no ponding water behind the dam and that the grades within the valley provide positive drainage into a new spillway added on the downstream side of the dam. Therefore, the decision was made to modify the dam by lowering the dam crest and constructing the new spillway and the energy dissipater for controlling and managing runoff. The old spillway was a vertical drop-down concrete shaft that horizontally extended through the dam and daylighted at the toe where it was monitored for total suspended solids (TSS). A new two-level spillway with an energy dissipater was constructed along the center of the dam to handle the 100-year storm and Probable Maximum Flood (PMF) events. The old spillway was abandoned with cement grout at the completion of the project. The dam modification design evaluated overtopping during construction as a concern for the loss of CCR, and a detailed construction sequencing was developed to contain the PMF during construction in order to satisfy the TSS discharge criterion. An important long-term design feature required managing the water that accumulates under the cover system against the upstream face of the dam. A separate subsurface drainage system was designed and constructed, in addition to the spillway drainage system. Overall, it was a unique and challenging project. The expanded abstract includes (i) spillway and energy dissipater hydraulic design; (ii) drainage system design; and (iii) dam lowering and construction sequencing.

Publication Summary

  • Geosyntec Authors: Matt Bardol, Dan Bodine
  • All Authors: Matt Bardol, Thad Penueal (Entact), Omer Bozok, Dan Bodine
  • Title: United States Society on Dams (USSD) 2022 Annual Conference
  • Event or Publication: Event
  • Practice Areas: Dams; Dam Safety
  • Citation: United States Society on Dams (USSD) 2022 Annual Conference and Exhibition at the Town and Country Resort in San Diego, California on April 11 through 14, 2022.
  • Date: Wednesday, April 13 at 5:05 p.m. PDT
  • Location: San Diego, California
  • Publication Type: Platform Presentation